1. Field of the Invention
The present invention relates to a digital TV receiver that receives a vestigial side band (VSB) signal and demodulates it, and more particularly, to a digital TV receiver and a method for receiving a digital TV signal, in which a symbol clock and a synchronizing signal are recovered from received data.
2. Background of the Related Art
Generally, a VSB transmission system of a Grand Alliance adopted as a standard of a digital TV transmission mode in USA and Korea modulates one side band signal of two side bands generated up and down around a carrier wave when the other side band signal is attenuated. That is, the VSB transmission system transmits one side band spectrum of a base band to a pass band to efficiently use a band region.
The VSB transmission system is more remarkable than other digital TV transmission systems in that it includes a pilot signal, a data segment synchronizing signal, and a field synchronizing signal. These signals can be used to improve carrier recovery characteristic and timing recovery characteristic. The recovery characteristic of the synchronizing signals greatly act on performance of the overall VSB system.
A transmitting party such as a broadcasting station transmits a signal through a mapper that acts to convert the signal to a desired power level. As an example, in case of 8 VSB for terrestrial broadcasting, an output level of the mapper is one of eight symbol values (amplitude level), xe2x88x92168, xe2x88x92120, xe2x88x9272, xe2x88x9224, 24, 72, 120, and 168. The mapper forcibly inserts a segment synchronizing signal of four symbols for each unit of 828 symbols by a protocol and forcibly inserts a field synchronizing signal into a 313rd data segment location.
At this time, the protocol of the segment synchronizing signal has a logic format of 1, 0, 0, and 1. The output level of the mapper is 120 when the synchronizing signal is 1 while the output level is xe2x88x92120 when the synchronizing signal is 0. That is, the segment synchronizing signal is repeated with two levels per data segment.
FIG. 1 shows a VSB data frame format including the data and the synchronizing signal. Referring to FIG. 1, one frame includes two fields while one filed includes 313 data segments. One data segment includes 832 symbols. In this case, first four symbols in one data segment correspond to a segment synchronizing portion, and the first data segment in one field corresponds to a field synchronizing portion.
FIG. 2 shows a configuration of the field synchronizing portion.
Referring to FIG. 2, a data segment synchronization of four symbols, a PN511 sequence which is a pseudo random sequence, three PN63 sequences, and VSB mode information of 24 symbols are provided, while other 014 symbols are reserved. In other words, the PN511 sequence includes 511 pseudo random symbols. The second PN63 sequence of the PN63 sequences has an inverted symbol configuration per field, wherein xe2x80x981xe2x80x99 is inverted to xe2x80x980xe2x80x99 while xe2x80x980xe2x80x99 is inverted to xe2x80x981xe2x80x99. Accordingly, one field may be divided into an even field and an odd field depending on polarity of the second PN63 sequence.
FIG. 3 is a block diagram illustrating a general digital TV receiver that receives and recovers the synchronizing signal. Referring to FIG. 3, if a radio frequency (RF) signal modulated in a VSB mode is received through an antenna 101, a tuner 102 selects a user""s desired special channel frequency. Then, the tuner 102 acts to transit a VSB signal of an RF band loaded in the channel frequency to a primary fixed intermediate frequency (IF) band of 44 MHz or 43.75 MHz and to filter other channel signals.
An output signal of the tuner 102 that acts to transit an arbitrary channel spectrum to the primary fixed IF band passes through a surface acoustic wave (SAW) filter 103 adapted to remove an adjacent channel signal and a noise signal.
At this time, since all information exist in a band of 6 MHz from the IF of 44 MHz, the SAW filter 103 removes all periods except for the band of 6 MHz with information from the output of the tuner 102 and then outputs a digital broadcasting signal to the down converter 104.
The down converter 104 performs down conversion of the signal filtered by the SAW filter 103 with an oscillation frequency for generating a second IF signal and outputs the converted second IF signal to a demodulator 105.
The demodulator 105 demodulates a VSB modulated signal in an analog mode and outputs the demodulated signal to an A/D converter 106.
The A/D converter 106 converts the VSB demodulated signal to a digital signal in accordance with an A/D clock provided from a timing recovery unit 107 and outputs the digital signal to the timing recovery unit 107, a synchronizing signal recovery unit 108, and an equalizer 109.
In this case, a VSB transmission system of an advanced television systems committee (ATSC) suggested by a US digital TV mode transmits a transmitting signal which includes data only.
Therefore, to recover the data in a receiving party shown in FIG. 3, the same clock as that used during transmission should be generated. The timing recovery unit 107 acts to generate the clock. In the currently suggested ATSC standard, the receiving party performs timing recovery using a data segment synchronizing signal regularly inserted from the transmission party.
Namely, the timing recovery unit 107 obtains a symbol timing error, which will be used as an A/D clock (A/D clk) of the A/D converter 106, using the segment synchronizing signal detected by the synchronizing detector 108.
The segment synchronizing signal is used to determine a start position of a segment and at the same time to recover timing of the system. Accordingly, the synchronizing recovery unit 108 detects a segment synchronizing signal and outputs the detected segment synchronizing signal to the timing recovery unit 107 and the equalizer 109. The timing recovery unit 107 performs timing recovery using the segment synchronizing signal. The synchronizing recovery unit 108 recovers the field synchronizing signal using the detected segment synchronizing signal and outputs the recovered field synchronizing signal to the equalizer 109.
However, since the segment synchronizing signal consists of four symbols 1001, it may be difficult to be detected due to a damaged channel having a large sized ghost even if it is repeated per segment. Particularly, under a ghost channel environment in which a serious null is generated around the segment synchronizing signal, it is more difficult to detect the segment synchronizing signal. Generally, in a downtown area where big buildings are located, or in an area where a VSB terrestrial broadcasting is received through an indoor antenna, it is well known that a serious near-by ghost signal is generated.
Therefore, if the segment synchronizing signal is not detected for the above reasons, timing recovery and field synchronizing signal detection are not implemented. In this case, data recovery may be delayed or may not be implemented as the overall systems as well as a channel equalizer do not work.
In other words, if the synchronizing detecting unit 108 does not detect the segment synchronizing signal, the timing recovery unit 107 cannot recover timing data. Since the timing data is provided to the A/D clock of the A/D converter 106, the segment synchronizing signal cannot be detected unless the timing data is recovered. Moreover, since the structure of FIG. 3 has many analog components, it is difficult to control the components.
Accordingly, the present invention is directed to a digital TV receiver and a method for receiving a digital TV signal that substantially obviate one or more problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a digital TV receiver and a method for receiving a digital TV signal, in which timing recovery and segment synchronizing signal recovery are independently implemented.
Another object of the present invention is to provide a digital TV receiver and a method for receiving a digital TV signal, in which VSB demodulation is digitally implemented.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a digital TV receiver according to the present invention includes: an A/D converter and demodulator converting VSB modulated signals to digital signals and demodulating the digital signals in a VSB mode; a resampler performing interpolation to reduce errors between the signals digitalized by the A/D converter and demodulator using a timing error of current symbols; a timing recovery unit obtaining the timing error of the current symbols from the output of the resampler and outputting again the obtained timing error to the resampler; an equalizer equalizing a signal included in the output of the resampler using segment and field synchronizing signals as training signals; and a synchronizing recovery unit recovering the segment and field synchronizing signals from the output of the equalizer and outputting again the recovered synchronizing signals to the equalizer.
The timing recovery unit includes a pre-filter that passes through a spectrum edge portion only from the output of the resampler, a timing error detector that detects timing error data using the signs of the two symbol samples and one intermediate sample that have passed through the pre-filter, a loop filter that passes through low band signal components only among the timing error data extracted from the timing error detector, and a numerically controlled oscillator (NCO) that controls a sampling timing of the resampler by converting the output frequency in accordance with the low band components of the timing error data.
The synchronizing recovery unit includes a correlation unit obtaining a correlation value between the received signal and a preset synchronizing signal, a maximum value position detector outputting a position value of a symbol having a maximum correlation value per field, a synchronizing lock signal generator checking reliability of the position value of the symbol detected by the maximum value position detector to generate a synchronizing lock signal, and a synchronizing signal detector detecting a segment synchronizing signal and a field synchronizing signal by obtaining a relative position value of the position value of the symbol having the maximum correlation value if the synchronizing lock signal is generated by the synchronizing lock signal generator.
In the present invention, timing recovery and segment synchronizing signal recovery are independently implemented, and VSB demodulation is implemented in a digital mode. This simplifies the system and improves stability and performance of the system, thereby allowing timing recovery and synchronizing signal recovery to be recovered on a channel to which a serious near-by ghost is applied.
In another aspect, a method for receiving a digital TV signal according to the present invention includes the steps of: a) converting VSB modulated signals to digital signals and demodulating the digital signals in a VSB mode; b) performing interpolation to reduce errors between the signals digitalized in the step a) using a timing error of current symbols; c) obtaining the timing error of the current symbols from the output of the step b) and outputting again the obtained timing error to the step b); d) equalizing a signal included in the output of the step b) using segment and field synchronizing signals as training signals; and e) recovering the segment and field synchronizing signals from the output of the step d) and outputting again the recovered synchronizing signals to the step d).
It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.